1. Field of the Invention
This invention relates generally to a system and method for determining whether the drop in voltage of a low performing fuel cell in a fuel cell stack is the result of anode reactant starvation and, more particularly, to a system and method for determining whether the drop in voltage of a low performing fuel cell in a fuel cell stack is the result of anode starvation by comparing the rate of the voltage drop to a predetermined threshold.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As a fuel cell stack ages, the performance of the individual cells in the stack degrade differently as a result of various factors. In addition, there are various stack operating conditions that cause the cells to operate differently. There are also various causes of low performing cells, such as cell flooding, loss of catalyst, etc., some temporary and some permanent, some requiring maintenance, and some requiring stack replacement to exchange the low performing cells. For example, if one cell is starved of reactants, especially hydrogen, the voltage for that cell will drop and undesirable side reactions could occur. It is known in the art that a low performing cell can be the result of loss of either or both anode reactants or cathode reactants to the cell, such as, for example, ice buildup in the reactant flow channels. As used herein, anode reactant starvation refers to a lesser amount of hydrogen reaching the cell than is necessary to sustain the drawn current and cathode reactant starvation means a lesser amount of oxygen reaching the fuel cell than is necessary to sustain the drawn current.
Although the fuel cells are electrically coupled in series, the voltage of each cell when a load is coupled across the stack decreases differently where those cells that are low performing have lower voltages. Because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Further, since the cells are electrically coupled in series, each cell must produce the full stack current. Thus, it is necessary to separately monitor the voltages of the fuel cells in a stack to ensure that the voltages of the cells do not drop below a predetermined threshold voltage to prevent cell voltage polarity reversal, possibly causing permanent damage to the cell.
Cell voltage monitors or stack health monitors are used to measure the voltage of the fuel cells in the stack to look for behavior in the cells indicative of problems with the stack. The cell voltage monitor generally includes an electrical connection to each bipolar plate, or some number of bipolar plates, in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell.
If the minimum cell voltage falls below some predetermined minimum cell voltage threshold indicating something is wrong with the cell, such as 300 mV, the system control will likely take some action in an attempt to prevent the minimum cell voltage from falling farther, which could damage the cell. Various remedial actions can be taken, such as providing a more favorable stack humidification, increasing the stack stoichiometry, etc. However, the main remedial action usually taken in response to a significantly low performing cell is to power limit or current limit the stack to prevent significant current draw from the stack. However, power limiting the stack has obvious drawbacks for vehicle operation and driver satisfaction.
It is desirable to limit the amount of catalyst provided on the electrodes in a fuel cell stack because of cost. Because less catalyst is required for the anode side reaction, the anode and cathode electrodes will typically have a different catalyst loading, where the anode catalyst loading, i.e., amount of platinum, is less than the cathode catalyst loading, which results in a different electrode double layer capacitance between the electrodes. The difference in the double layer capacitances causes a different voltage in response to changes in stack current resulting in the cathode voltage falling more slowly than the anode voltage. Typical values for the anode electrode double layer capacitance is in the range of about 1-5 F/cell and for the cathode electrode double layer capacitance is in the range of about 10-40 F/cell. This means that when the cell is totally starved of reactants at a given stack current density, the cathode electrode will change its voltage much slower than the anode electrode.
It is known in the art that a low performing cell as a result of cathode reactant starvation does not harm the stack, but a low performing cell as a result of anode reactant starvation does damage the electrode catalyst resulting in cell damage. If a particular fuel cell is not receiving enough oxygen for the reaction, hydrogen pumping occurs where hydrogen from the anode side is drawn to the cathode side through the cell membrane. However, this reaction does not cause any damage to the cathode electrode. Anode starvation causes a reaction on the anode side that corrodes the carbon support in the catalyst layer of the electrode causing the potential of that electrode to go high relative to the standard hydrogen electrode (SHE), but in an opposite direction than the stack voltage, which can result in a negative cell voltage. Since there is typically less catalyst to lose on the anode and the corrosion effect is a linear one with current density, the loss of catalyst as a result of carbon support damage has a more dramatic impact on the anode side.
The above described phenomenon can be described in more detail as follows. Because electrodes can be starved but still be fed a portion of the stoichiometric reactants, some of the current will be carried by their normal fuel cell reactions, namely, for the anode H2→2H++2e− and for the cathode 2H++2e−+½O2→H2O. If the cathode is starved of oxygen, eventually a non-damaging hydrogen pump would be created, where 2H++2e−→H2. If the anode is starved of hydrogen, the current will eventually be taken up by non-damaging oxygen evolution and damaging corrosion of the anode carbon support. The ratio of the two is determined by the anode catalyst and electrode composition and the potential of the electrode, where for the anode ½C+H2O→2H+2e−+½CO2 or H2O→½O2+2H++2e−.